Good morning, John
So, if I understand correctly, it is the considerable dihedral of
Ercoupe wings (and attached tanks) and the extended length of the 15
gallon tanks that requires the necessary vent line to connect these
tanks at the high end such that when the tanks are full fuel cannot
block the interconnection. That makes sense and would seem to
represent a "perfect world" approach.
The Ercoupe fuel system with unvented caps accepted by the CAA for use
on a production aircraft under CAR 3 in the normal category, which
vents wing tanks from the inboard end (well below the fuel level of
full wing tanks), would have been excepted from your CAR 4a.606 rule by
virtue of the engine being fed by gravity from the nose tank and not
directly by a fuel pump. Even so, wing tank fuel flow "problems" have
been encountered.
It seems to me that the way the two wing tanks interact while fuel is
drawn is unchanged by the presence or absence of a fuselage tank so
long as a line from the top of each tank at the inboard end connect
(high) inside the fuselage before venting to the outside. Given that
the fuel may be consumed unevenly, perhaps exclusively from one of the
two wing tanks, until that "...tank draws down to the level of the
line..." in the Ercoupe the effect would be a slight wing heaviness on
the side of the tank not supplying fuel. Neither tank could empty
(such that engine operation would be affected) before the pressure
differential between the tanks relieves itself. While such relief
might occur in "burps", progressively extended periods of a "clear"
interconnection between the two tanks, so long as the tanks can
equalize fully and reliably with appropriate reserve fuel remaining we
know that if flight continues beyond that point that "... both tanks
are emptied at about the same time".
The reason that makes this "...not good enough for main tanks"
supplying a fuel pump/pressure carb system is what eludes me. If such
a system poses a credible and incipient hazard I have been unable to
envision it.
WRB
--
On Aug 18, 2010, at 09:55, John Cooper wrote:
On 8/17/2010 8:02 PM, [email protected] wrote:wouldn't simply venting
both tanks connect the air spaces through the atmosphere?
That works for our normal (small) wing tanks.
If you remove the header tank, then it becomes an issue. On the
surface, it would seem that if both tanks are vented to the
atmosphere, that should be sufficient. However, as others have said,
there are subtle differences that can crop up between the venting of
the two tanks, like leakage of one of the cap gaskets, differences in
airflow around the two cap vents, etc. It only takes a small pressure
differential to have a big affect on flow rates. Anyone who has a lot
of time working on Cessnas that draw from both tanks simultaneously
will tell you that, even if all the requisite plumbing is in place,
you can still have problems. There is a vent tube located behind the
lift strut on one side, and miss-positioning it by as little as 1/4"
can make the difference between normal operation and running out of
fuel with a full tank on one side.
The required connection between the airspaces is intended to
overwhelm any potential differences in tank venting simply by virtue
of the fact that the connection is bigger than the vent(s). This
works fine, and tank pressures are equalized, as long as there is no
slug of fuel trapped in the line (sort of reverse vapor lock...). The
easiest way to insure against this is to have the line level or
inclined downward toward both tanks, and enter the tanks above the
normal fuel level. Unfortunately, on the Ercoupe family of planes,
this is not practical as such a line would enter the cabin an inch or
two above the wing. If you connect the two inboard ends of the tanks
together, the lines enter below the level of the fuel in the tank(s)
and therefor do not connect the airspaces until at least one tank
draws down to the level of the line. Once this happens, things
generally start to equalize and ultimately, both tanks are emptied at
about the same time. Fine for transfer tanks but not good enough for
main tanks.
From CAR 4a.606:
If two or more tanks have their outlets interconnected they shall be
considered as one tank and the air space in the tanks shall also be
interconnected to prevent difference in pressure at the air vents of
each tank of sufficient magnitude to cause fuel flow between tanks.
Mechanical pump systems shall not feed from more than one tank at a
time except by special ruling from the Administrator.
(Note, the last statement describes the 30 gallon fuel system STC...)
CAR 3 and Part 23 contain similar wording.
--
John Cooper
Skyport East
www.skyportservices.net
